QSC 8.3 Fuel Pressure Loss at High RPM

[jjjlee1]

We have a 48 Sabre with QSC 8.3 engines and Zeus pods. Following some maintenance work on the pods and replacement of the Racor 1000 and Secondary Fuel Filters (FF5488), the Port engine (Serial number 73820594 upgraded to 600 HP) would not accelerate past 2000 rpm without falling back to 1800 rpm. The Vessel View generally reported a “low fuel pressure alarm” when the rpm dropped. The Starboard engine would accelerate under load normally. Our usual cruise is at 2550 rpm with 80% load and throttle, and WOT for the engines is 3065 rpm.

Initially we switched the Port Racor filters and changed the Port secondary fuel filter. We managed to get the Port engine up to 2500-2700 rpm but that was it. Based on suggestions in old sbmar.com posts, I installed 5-200 psi gauges at the inlet to the secondary filters. The Starboard engine shows ~6 psi with the lift pump  and 140 psi when the engines are started and the gear pump is operating. Under increased load the starboard pressure gauge raised slightly to 145 psi. On the port side. the lift pump showed ~8 psi and when the engine was started, the gear pump produced 130psi. Under load the Port pressure reading dropped to 120 psi and stayed there until around 2400-2500 rpm when the pressure dropped to 110 psi. At 2600 rpm the pressure drops to 100psi and at 2700 rpm, the pressure falls significantly and becomes erratic (40-80 psi).

Since it seemed to act like a fuel supply issue, we examined the interior of the secondary filters, rebuilt the Dual Racors, changed out the Racor outlet vacuum gauge and checked most of the fittings for tightness. The Secondary filter looked clean. While there was a small amount of “biological growth” captured in the Racors it appeared they were doing their job. Tests on the old vacuum gauge lead me to believe it was functioning properly (but we replaced anyway). I didn’t locate any loose fittings. A followup sea trial showed the same results as above.

It has been suggested that the fuel take inlet could be partly restricted as there apparently is a historic issue of a fuel tank inspection port gasket degrading which on rare occasions has caused a blockage on the fuel inlet. I can backflush that with compressed air but the restriction seems improbable since a restricted fuel line should show up on the Racor vacuum gauge (which it doesn’t). Does that sound right?

I know the boat has some history on the Port engine fuel. The port fuel tank got some water in it 2 years ago; the boat yard replaced the lift pump twice but was able to “clean up minor corrosion” in the gear pump (so the gear pump was not replaced). We have added 100 operating hours since then. We even checked full WOT after some transmission work but before before the most recent minor maintenance work and new rpm problems. Any recommendations on next steps?

Topic

[jjjlee1]

More Information on problem

Port and Starboard pressure measurements at inlet to on-engine filter:

Key Off: 0 for both

Key On and after engine started: see previous post

No white or black smoke observed at any time from port engine

Port dual Racors have a new Parker Racor vacuum gauge installed. The highest Racor reading recorded during the tests of the previous post was 1-2″ Hg vacuum.

After reading through related low fuel pressure posts, it makes sense that the port side pressure readings after the engine is started are lower than the starboard side (since the engine mounted fuel filter is on the inboard side with 6-7 foot tubing runs back to the mechanical low pressure pump and the fuel injection pump. The tubing looks to be about 3/8″ diameter and has some tight 90 degree bends. This didn’t seem to be an issue before the recent problems.

 

[jjjlee1]

Thanks Tony for the quick reply. The 0-200 psi pressure gauge referenced in my earlier post is installed as shown in your picture (at the inlet port to the engine mounted 2 micron filter). We also installed a tee with a valve on the pressure gauge riser so that we could bleed air and/or check pumping volumes of the electric pump. As suggested, the gauges are installed semi-permanent/safe. I installed the same setup on both the Port and Starboard secondary filter inlets so that I could compare performance between the two engines. The electric “lift pump” measurements were taken as requested (key on, engine off): Starboard key on, engine off: about 6 psi (hard to see on the 0-200 gauge); Port key on, engine off: about 8 psi.

The Engine-on (at 600 rpm) measurements were Starboard 140 psi and Port 130 psi. 

While the vessel was underway, the Starboard pressure reading increased slightly to 145 psi  (at 2500-2700 rpm) and was rock steady at all rpms. The Port pressure readings while the vessel was underway decreased to 120 psi and stayed there until around 2400-2500 rpm when the inlet pressure dropped to 110 psi. At 2600 rpm the pressure drops to 100psi and at 2700 rpm, the pressure falls significantly and becomes erratic (40-80 psi). If we tried to push the port engine beyond 2700 rpm, we would get a low fuel pressure alarm.

[Tony Athens]

Until you check things this way, you are just guessing what the issue is. Make the gauge semi-permanent /safe until you sort it out.  

QSC

 

Key off=ZERO

 

Key on, electric pump working,  engine off, about 10 PSI

 

Engine on, close to 100PSI

 

Fuel Pressure Test Port

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